53.7.3 External Real Time Oscillator

The low-frequency crystal oscillator is optimized for use with a 32.768 kHz watch crystal. When selecting crystals, load capacitance and the crystal’s Equivalent Series Resistance (ESR) must be considered. Both values are specified by the crystal vendor.

The SAM C20/C21 oscillator is optimized for very low-power consumption, hence close attention must be paid when selecting crystals.

The typical parasitic load capacitance values are available in the Electrical Characteristics section. This capacitance and PCB capacitance can allow using a crystal inferior to 12.5 pF load capacitance without external capacitors as shown in Figure 53-9.

Figure 53-9. External Real Time Oscillator without Load Capacitor

To improve accuracy and safety factor, the crystal data sheet recommend adding external capacitors as shown in Figure 53-10.

To find suitable load capacitance for a 32.768 kHz crystal, consult the crystal data sheet.

Figure 53-10. External Real Time Oscillator with Load Capacitor

Table 53-6. External Real Time Oscillator Checklist
Signal Name	Recommended Pin Connection	Description
XIN32	Load capacitor 22pF⁽¹⁾⁽²⁾	Timer oscillator input
XOUT32	Load capacitor 22pF⁽¹⁾⁽²⁾	Timer oscillator output

1. These values are only given as typical examples.

2. The capacitors must be placed close to the device for each supply pin pair in the signal group.

Note: In order to minimize the cycle-to-cycle jitter of the external oscillator, keep the neighboring pins as steady as possible. For neighboring pin details, refer to the section “Oscillator Pinout”.